This invention relates to methods and apparatus for directional control of an aircraft during landing rollout (i.e., during the ground portion of the landing procedure wherein the aircraft decelerates from the touchdown velocity to a taxi or turn-out speed). More specifically, this invention relates to automatic control systems and methods utilized therein for maintaining an aircraft within the lateral boundaries of a runway when a crosswind landing is executed on a runway that exhibits reduced frictional properties because of environmental factors such as rain, snow or ice or because of other deleterious runway conditions or, in some cases, on runways of less than the customary width.
As is well known in the art, crosswinds and adverse conditions that reduce the coefficient of friction exhibited by the runway surface often interfere with or limit the operation of both military and commercial aircraft. In particular, depending on the severity of the crosswind and the runway conditions, the width of the runway, the type of aircraft involved, and various other factors, it may be impossible to land at an intended destination while maintaining adequate safety margins. In such a situation, the aircraft often must be diverted to an alternate landing site or its landing delayed until wind and/or runway conditions improve to a point that a landing attempt is well within the operational capabilities of the aircraft and its crew. Needless to say the resulting delays and diversions are costly in terms of fuel and time expended and can cause interruptions in scheduled aircraft operation as well as interference with the tactical deployment of military aircraft.
Several factors contribute to the problem of maintaining directional control when executing a crosswind landing on a surface having a relatively low coefficient of friction. First, in order to achieve a relatively straight-in landing approach, the aircraft must be flown at a particular crab angle or flown with the windward wing down so as to establish aerodynamic slip. Often times the pilot must make corrections during the final phases of a landing approach to account for changes in the aerodynamic state of the aircraft such as those resulting from crosswind variations, increasing ground effects or other environmental factors. In addition, a de-crabbing or reduction of aerodynamic slip maneuver is generally executed immediately prior to touchdown in an attempt to align the aircraft with the centerline of the runway as the landing rollout is initiated. If these maneuvers are not properly executed, a rather complicated sequence of precise control actions may be required to maintain the aircraft on the runway. Moreover, even if the aircraft touches down properly and the pilot and crew are aware of runway conditions prior to landing, maintaining directional control at touchdown and during the initial phase of the landing rollout requires that the aircraft pilot anticipate the positional perturbations and effects of several factors and forces including the crosswind and pilot asserted commands to the aircraft rudder, steering and braking systems. Because of the relatively high landing speed of most modern aircraft, the pilot has little time to institute a control action during the initial phase of landing rollout (primarily operation of the rudder) and little time is available to evaluate the results of an asserted control action and institute any required corrective measure. Thus, a consideration of only those factors that influence directional control at touchdown and during the initiation of the landing rollout reveals several restrictions and limitations that have prevented modern high speed aircraft from attaining true all-weather operating capability.
Further, even though the landing roll begins in a manner which allows adequate initial directional control and guidance of the aircraft within the lateral boundaries of the runway, it can be difficult to maintain directional stability as the aircraft is decelerated to a speed at which it can be turned off the runway and stopped in the desired manner. In this regard, as the aircraft decelerates the aerodynamically generated lift forces rapidly decrease and the weight of the aircraft is transferred to the landing gear. As aircraft velocity decreases, the yawing moment and lateral force produced by a given amount of rudder deflection decreases thereby lessening the directional control available through operation of the rudder until a speed is reached wherein the rudder is substantially ineffective in steering the aircraft along the runway. Since the transfer of weight to the aircraft landing gear that accompanies deceleration increases the directional control capability of the aircraft nose wheel, or other steerable portion of the landing gear, directional stability and control can be maintained by simultaneously controlling the rudder and aircraft steering while the aircraft passes through the midrange or transitional velocity portion of the landing roll. As is the case with the initial, high speed portion of the sequence, the pilot's reaction time and inability to rapidly determine and assert the exact control required with unerring accuracy has limited crosswind landings to situations wherein the combined crosswind and runway surface condition permits rather wide margins of error.
The difficulties encountered in crosswind landings on relatively narrow runways or a runway of substantially reduced surface friction does not end as the aircraft brakes to a relatively low speed. In particular, the heavy braking and, in some cases, wheel lock-up that occurs during this phase of the landing rollout substantially reduces the frictional contact force between the aircraft tires and the runway surface. This decreases the lateral force capability of the landing gear thereby rendering the aircraft even more susceptible to lateral displacement under the force of crosswinds. Moreover, employment of various conventional deceleration devices such as thrust reversers and drag chutes have a marked tendency to cause the aircraft to yaw into the wind (i.e., "weathervane"). Thus, employment of such devices during a crosswind landing procedure on a runway that is exceptionally narrow or slippery present further and compounded control problems.
Most prior art attempts to deal with the numerous problems associated with executing a crosswind landing have been limited to the airborne portion of the landing sequence and, in particular, to generating command signals which direct the pilot (or autopilot system) to execute maneuvers immediately prior to touchdown that will align the aircraft with the runway centerline. In addition to such attempts, which are of no assistance relative to steering the aircraft during the landing rollout, the prior art includes an apparently limited number of proposals for automatically controlling one or more aspects of the landing rollout sequence. For example, U.S. Pat. Nos. 2,762,006 and 2,762,007, which issued to A. W. Blanchard, disclose a system wherein control over the aircraft ground roll trajectory is primarily based on rudder control during high speed portions of the landing rollout and on control of the steerable wheels of the aircraft during the lower speed portions of the sequence. Such prior art may prove satisfactory relative to assisting the pilot during landing rollout under typical landing conditions, but cannot and does not include provisions for exercising the precise control over the aircraft rudder, steering and braking systems that is required under combined conditions of crosswind and relatively low coefficient of runway surface friction (i.e., runways that are wet or coated with snow or ice).
Further prior art proposals such as U.S. Pat. No. 4,006,870, issued to Boone et al, for guiding an aircraft through the landing rollout sequence are not only limited to crosswind landings on relatively normal runway surfaces, but can only be used on the airfields equipped with special radio equipment. In particular, such systems utilize localizer signals similar to those employed in generating the localizer signal of a conventional inbound landing system (ILS).
Accordingly, it is an object of this invention to provide a ground roll control system which is useful during execution of a cross wind landing on a runway of relatively low surface friction or an extremely narrow runway.
Further, and more specifically, it is an object of this invention to provide a control system which coordinates rudder, steering and brake operation to achieve substantially improved directional stability and control throughout all phases of a landing rollout under conditions of cross wind and reduced runway surface friction which lie outside the operational capabilities of an aircraft equipped with a prior art landing control systems.
It is an additional object of this invention to provide a landing rollout control system of the above-mentioned type which does not require ground-generated localizer signals or any other specialized ground-based equipment.